During testing of a device-under-test (DUT) using a number of test instruments, it is sometimes desirable to carry out tasks on the various instruments that are synchronized in time. Examples of such tasks are those required for measuring the root-mean-square (RMS) power of a DUT in response to a stimulus signal. To make such a measurement in the prior art, a test system 2 (FIG. 1) including a function generator 4 and two digital multimeters (DMMs) 6, 8 connected to a controller 10 is required. The controller 10 controls and coordinates the operation of the function generator 4 and the DMMs 6, 8. To make a measurement, the controller 10 first configures the function generator 4, for example to select an output waveform type, and to set the amplitude and frequency of the output signal. The controller 10 next instructs the function generator 4 to output the selected signal for use as a stimulus signal to the DUT 12. The controller 10 then polls the function generator 4 to find out when the output signal of the function generator 4 is stable. The time taken for the output signal to stabilize depends on the frequency of the signal. Once the function generator 4 informs the controller 10 that the output signal is stable, the controller 10 will then instruct a first 8 of the two DMMs to measure the RMS voltage. After the controller has obtained the RMS voltage reading from the first DMM 8, the controller 10 instructs the second DMM 6 to measure the RMS current. When the controller 10 receives the current reading, it then computes the RMS power. One disadvantage of such a solution is that the voltage and the current measurements are not carried out at exactly the same instant in time but a short interval apart. Such a time lag in making the current measurement may result in inaccuracies in the power measurement. Another disadvantage is that time is wasted in the communication between the controller 10 and each of the function generator 4 and the DMMs 6, 8 during the measurement.
With the introduction of LAN extensions for instrumentation (LXI), test and measurement instruments are able to communicate directly with one another via multicast messages to perform time-based measurements. All the instruments will be able to send time-stamped data to one another. And with the IEEE 1588 time synchronization protocol, any discrepancies in the clocks in the different instruments can be reduced to less than 50 ns. With such LXI instruments, the above described power measurement on the DUT 12 may be carried out by the DMMs 6, 8 at a predetermined time without the need for the controller. However, this predetermined time is dependent on the time it takes the output signal of the function generator 4 to stabilize. Since the time taken for the output signal to stabilize is dependent on the frequency of the output signal, the DMMs 6, 8 are programmed with the longest time taken for any output signal to stabilize. This solution for making measurements at predetermined fixed times is thus not optimal for use in a production line where throughput is of utmost importance and where the arrival of a DUT at the test system is sometimes unpredictable.